The development of T lymphocytes in the thymus is dependent upon antigen receptor-mediated induction of differentiation and survival in a process termed positive selection. Positive selection also encompasses lineage diversification, resulting in the production of CD4 and CD8 T cells from a common precursor. It is known that the MAPK/Erk signaling pathway plays a critical role in many aspects of this process. We are particularly interested in how the kinetics and magnitude of MAPK activation are regulated and how this signaling pathway in turn regulates gene expression associated with positive selection and lineage commitment. We have previously identified a gene encoding a novel cell surface protein with likely signaling capability that itself is regulated by MAPK in the thymus. This protein is a member of the immunoglobulin superfamily, contains tyrosine based signaling motifs in the cytoplasmic tail and is conserved in humans. Aim 1 proposes to analyze the biochemistry and function of this novel protein in the context of T cell development. In Aim 2 we will analyze the stage of the double positive thymocyte that is susceptible to differentiation signals, the role of MAPK in inducing functional maturation of thymocytes, and use a biochemical definition to time the establishment of lineage commitment in isolated thymocytes. This information will be used to identify additional genes that are targets of MAPK and that encode proteins involved in positive selection. Aim 3 focuses on one mechanism of regulation of MAPK signaling, that is, inactivation by dual specific phosphatases. Some of these enzymes are induced as immediate early genes and localize to the nucleus. We propose to analyze the pattern of expression of these phosphatases in vivo and in isolated thymocytes in the process of differentiation, and to search for novel members of this family. Both in vitro and in vivo approaches are outlined to determine the specific role of these enzymes in T cell development. Finally, we will take advantage of the properties of a cytosolic member of this family of phosphatases to analyze the specific role of MAPK nuclear localization in T cell development.